1. Field of the Invention
The present invention relates to a power transmission unit for automotive vehicles, and more particularly to a shift mechanism for a change-speed gearing in a power transmission unit which includes a plurality of fork shafts slidably supported from the transmission casing of the unit through a shift restriction mechanism in such a manner that any one of the fork shafts is selectively shifted by a shift-and-select lever along the fore-and-aft axis of the vehicle while the remaining fork shafts are locked in their neutral positions.
2. Description of the Prior Art
In a conventional shift mechanism a reverse shift fork member is operatively connected to a reverse fork shaft selected from the fork shafts, the reverse shift fork member being arranged to shift a reverse idler gear toward and away from reverse drive and output gears for establishing a reverse gear train, a shift head member is fixed to a rearwardly extended portion of the reverse fork shaft, and the shift-and-select lever is arranged to be engaged with the shift head member in its selecting operation and to shift the reverse fork shaft in the forward direction so as to establish the reverse gear train. In use of the shift mechanism, if a forward inertia force acts on the fork shafts in an occurrence of collision of the vehicle with the shift-and-select lever in its neutral position, the reverse fork shaft is displaced in the forward direction due to additional forward inertia forces acting on the reverse idler gear and the reverse shift fork member. As a result of the forward displacement of the reverse fork shaft, the reverse gear train is unexpectedly established, and the shift restriction mechanism acts to lock the remaining fork shafts in their neutral positions. This means that even if the collision occurs at a relatively low speed without any obstruction to re-travel of the vehicle, it is unable to conduct the shifting operation of the shift-and-select lever for re-travel of the vehicle.